Process for producing fine-grained highly oriented silicon steel



Jan. 18, 1955 c. G. DUNN 2,700,006

PROCESS FOR PRODUCING FINE-GRAINED HIGHLY ORIENTED SILICON STEEL FiledSept. 3, 1955 2 Sheets-Sheetl His flfforngy.

Jan. 18, 1955 c. G. DUNN 1 2,700,006

PROCESS FOR PRODUCING FINE-GRAINED HIGHLY ORIENTED SILICON STEEL FiledSept. 5, 1953 2 Sheets-Sheet 2 Fig.

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\1 a in 'fl renfor' Ceci/ G. Dunn Qy m/f His fllforn q United StatesPatent PROCESS FOR PRODUCHWG FINE-GRAINED HIGHLY ORIENTED SILICON STEELCecil G. Dunn, Pittsfield, Mass, assignor to General Electric Company, acorporation of New York Application September 3, 1953, Serial No.378,311 Claims. (Cl. 148- -12.5)

This invention relates to magnetic materials. More particularly, itrelates to a method of producing silicon steel magnetic strip materialhaving a very high degree of preferred orientation.

Because of the anisotropic character of magnetic permeability u withcrystallographic direction in single crystals of iron and other magneticmaterials, it is desirable to align certain crystallographic directionsof the crystals in any such material in such manner as to give the.highest possible permeability u for a given flux density B or converselyto attain as high a flux density B as possible for a given appliedmagnetic field H where B=uH.

It is an object of this invention to provide a process for makingmagnetic strip material having a very high degree of preferredorientation.

Another object of the invention is to provide a process wherein themagnetic orientation of an entire strip of silicon steel is modeledafter a single seed crystal.

A further object is to provide very thin strips of highly orientedmagnetic material.

Other objects will become apparent from a consideration of the followingdescription and the drawings in which Fig. 1 represents a number ofcrystals grown in one end of a silicon steel strip; Fig. 2 illustrates amethod of isolating a seed crystal; Figs. 3-6 illustrate thereorientation of an actual seed crystal; Figs. 7 to 9 depict thereorientation of an idealized cubic crystal; Fig. 10 is a quantitativepole figure of a specimen having a desired orientation after a oneminute anneal of 980 C.; and Fig. 11 is a graph giving the results of amagnetic torque test .of three differently oriented specimens subjectedto the same annealing treatment.

'It is known that large crystals may be produced by feeding a metal,which has preferably been strained in tension, slowly into a furnacewhose temperature is high enough to produce recrystallization in'thesample. Likewise, different methods of rolling and heat treatingmagnetic steels have resulted in varying degrees of success in aligningthe metallic grains of metal in such directions crystallographically asto give better magnetic properties. An example of the latter is theorientation (110) [100] for silicon steel col-d rolled strip, the (110)notation denoting a (110) crystallographic plane in the plane of thepiece and the [100] notation denoting a crystallographic direction ofthe form [100] parallel to the rolling direction. The ultimate favorablemagnetic condition would be attained in material comprising a singlecrystal which is oriented in a selected and predetermined manner. Incommercial production this single crystal is difiicult to obtain due tothe fact that upon recrystallization crystals having a variety oforientations are formed.

While fairly thick strips of single crystal material have been formed,attempts to produce long strips of single crystal material thinner than0.005 inch in thickness have been unsuccessful.

It has been found that if a single crystal of silicon steel strip iscold rolled and recrystallized under carefully controlled conditions thefinal magnetic orientation of the strip is no longer that of a singlecrystal but approaches very closely to what it would be if the entirestrip were composed of a single properly oriented crystal.

Briefly stated, in accordance with one of its aspects, this inventioncomprises isolating a crystal at one end of a single silicon steelstrip, cutting away other crystals from this end to leave the crystalconnected by a thin neck to the remainder of the strip, orienting thecrystallo- ICC ' graphic directions of the crystal into [001] orien* atemperature of 900-1000 C., moving the strip into the furnace at a rateequalto the rate of growth of the crystal, cold rolling the strip tobring about a reduction in thickness of about 70 per cent, andrecrystallizing the strip at a temperature of about 980 C. in a reducingatmosphere. It has been found that the conditions for transforming apiece of magnetic material into a single crystal of desirableorientation and subsequently converting a large portion of stripmaterial to which the crystal is attached to approximately theorientation of the seed crystals are as follows: First, the matrix ormaterial to be transformed must be able to support the growth of a grainor crystal of the specified orientation. Second, one or more grains mustbe grown at one end of the strip and a seed crystal selected andisolated from the rest of the strip except for a small neck or matrixmaterial which connects it to the main part of the strip. Further, theseed crystal must be placed in the desired orientation with respect tothe strip proper. Third, the sheet or strip with the reoriented seedcrystal must be fed into a furnace at a speed corresponding to thenatural rate of growth of the selected seed crystal for a temperature ashigh as possible in a safe range where other grains will not have timeto start their growth. Fourth, the single crystal thus produced is coldrolled to reduce its thickness by about 70 per cent. Fifth, the strip issubjected to a recrystallization treatment at a temperature of about 980C.

In carrying out the inven' on, strip material of any convenientthickness may be used as a starting material. The end of the strip 1 inthe drawing is placed in a furnace to grow a number of grains such as A,B, C, and D in one end thereof. Upon the basis of X-ray analysis acrystal such as B is selected as the seed crystal. The sample is thencut as indicated by dotted lines 2 and 3 in Fig. 2 leaving the opengrain boundary 7 of B in the sample connected to the main sample by aneck 4. This cutting may be accomplished by etching with a suitablereagent using a stopoff lacquer to control the process, or, when donemechanically, residual edge strains in the sample left by cutting may beremoved by etching off about one mil of the edges by an. etchingtreatment, the faces of the sample being protected by a layer oflacquer.

Having isolated a good seed crystal B as shown in Fig. 3, the next stepis to place the crystal B in any desired orientation with respect to themain strip by deforming or twisting neck 4 at red heat according to thechange in orientation required. In carrying out the deformation, it ispreferred to reorient crystal B by making two successive angularrotations of neck 4 at red heat, one rotation being about an axis TUnormal or perpendicular to the plane an axis MP lying in the plane ofthe strip. The axis MP is chosen in advance of reorientation accordingto the original orientation of the crystal and the final orientationdesired and need not be perpendicular to the long direction or rollingdirection RD of the strip. The two rotations may be carried out in anyorder desired, but for the purpose of illustration the rotation aboutthe MP axis will be considered as being made first. Starting with thestrip 1 as shown in Fig. 5 the crystal B is rotated through some angle(after heating neck 4 to red heat to make it deformable) which willbring one direction of the crystal into the desired orientation. Thecrystal position after this rotation is shown in Fig. 6. The secondrotation about an axis such as TU normalto the strip 1 is best shown inFigs. 3 and 4. In Fig. 3 the crystal B is shown as being removed fromits desired direction about TU by an angular amount equal to 0. With theneck 4 at red heat the crystal B is rotated about TU as an axis to bringthe crystal into the position shown in rection 5 which is perpendicularto the (110) plane, is

is used-in place of the-plane-of the "sample to simplify t the drawing.The first step is to rotate the crystal about MP through angle so thatthe [110] direction 5 coincideswithgthenormalfi to ;the:sample as,shownin Fig'ug.

Thisyoperation'; brings the (ll0)-plane in the,.plane of theitsample,but.thew[001-l direction may still beout of linc;.;.with.= the rollingdirection; :RD; ,bysome; angular amountgti. To bring [001] into aposition parallel to RD the-crystal B is rotatedabout the normal 6,which now coincides with the [110]- direction 5, an amount 0 to attainthecondition shown in Fig. 9. int-his final (110) [001-1 position the[110] direction is shown asbeing vertical,.which,means'that the (110)vplane lies in the planeof the strip and the [001] direction is parallelto the-rolling direction RD.

Having obtained the desired orientation of the seed crystal, the-sampleis fed intoa temperature gradient furnace maintained at a temperaturewhich will support the growth of the seed crystal but which will notstart the growth of differently oriented crystals and at such speed asis equal to the natural rate of growth of the crystal for the,temperature.

In actualoperation several specific silicon steels have been transformedinto desired orientations. However, it is emphasized that the processdescribed herein .is not to be limited to specific compositionsbutincludes all magnetic materials whether of face centered or bodycentered crystal structure, inasmuch as one skilled in the art, will beable using the teachings described herein, so to convert magneticmaterials of other compositions into. a form having a very high degreeof preferred crystal orientation.

The single crystal thus produced may have a thickness ranging from about0.005 to 0.050 inch. It is to be emphasized that crystal growth tookplace in a strip material already rolled to size. If the material hadbeen rolled to a thickness of less than 0.005 inch a single crystalwould'not have emerged from the subsequent heat treatment. In order toconfer the maximum possible degree of magnetic orientation to magneticmaterial having a thickness of lessthan 0.005 inch a single crystalstrip about 0.005 inch in thickness prepared as described above is coldrolled to reduce its thickness by about 70 per cent. It is thenrecrystallized at a temperature of about 980. C. These last cold rollingand crystallizing steps maybe taken in two or more stages starting withthicker single crystal material.

Magnetic material produced in accordance with the above described methodis not a single crystal but has a preferred crystal orientation closelyapproaching that of a single crystal. A quantitative pole figure of thedesired final product is illustrated in Fig. 10. If a final productpossessing the most desirable preferred orientation is to be producedclose control over the steps of the process must be maintained. This isparticularly important with respectto step 2 dealing with orientation ofthe seed cryswere given-different orientations as follows: specimen- S1had a tilt ofless than 3 degrees of the [001] direction up from therolling direction; specimen 82 had a tilt of 13 from-this direction;specimen S3 had a tilt of 25 from this direction. All three specimenshad a deviation of [110] from the cross rolling direction of less than1%". From Fig. 11 it may be seen that specimen S1, whichhad the leasttilt, was far superior to the other twoand; that specimen S3, which hada tilt of 25, gave a very unsatisfactory test.

If deviating primary recrystallization grains and secondaryrecrystallization grains are to be avoided it is importantthat the finalrecrystallization temperature be kept below l000- C. and preferably at atemperature of about 980 C. I have found that when primary recrys--tallization producesa high percentage of primaries in one preferredorientation and the remaining'primaries in other orientations in such away that a number of theelatter have diameters 2 or 3 times the average,then the; large primariesin deviating orientation grow into secondaries.

talsthequality of the final product is lower. Secondary,recrystallizationproceeds at a low rate below =l0OO C.

If therecrystallization conditions favor thegrowth of deviating primarycrystals-and secondarycrysand at a rapid rate near 1100 C. The inductionperiod with rising temperatures. On the other hand, primaryrecrystallization occurs slowly-below 600 C. and fairly rapidly near 700C. Thus, a recrystallization temperature of about 980 C. producesoptimum results. However, it is emphasized that.lower.recrystallizationtemperatures do not have an adverse efiect on the quality of thefinalproductand a temperature of 700 C. is satisfactory for therecrystallization step.

One. material used in my process was a silicon steel of from 3.0 percent to 3.5 per cent silicon content which was cold rolled to a finalthickness in the range 0.005 to 0.025 inch by a two-stage-cold trollingprocess froman initial thickness of about 0.1 inch with an intermediatecontinuous annealaat 850 C. between stages. A group of crystals weregrown as described above in one end of the piece and one of the bestcrystals selected by X-ray methods and isolated with a neck connectingitto the main strip as outlined.

at a rate of 0.2 to 2.0 inches per hour which alsorepresents the rate ofgrowth:of the single crystal of preselected orientation.

The materialundergoingtreatment may be strained either by being pulledin tension or by being cold rolled A reduc-v prior to the transformationto a single crystal. tion in thickness. in the range 2% to 6% ispreferred for a cold rolling strain.

While the present invention has been-described withreference'toparticular embodiments thereof, it will be understood that numerousmodifications may be made. by those skilled in the art without actuallydepart1ng Therefore, I aim the appended claims to cover all suchequivalent variations as come;..

from the invention.

within the true spirit and scope of the foregoing disclosure.

What I claim as new and deslre to secure by Letters Patent of the UnitedStates is:

l. The method of producing thin oriented magnetic} stripswhich comprisescold rolling a s1l1con-s teel-str1p'. of an initial thickness of about0.005 to 0.025 mch by a two-stage process with an intermediate anneal at850 C. betweenstages, heating one end of the cold rolle d silicon steelstrip to grow a number of crystals in 821d end, selecting oneof saidcrystals as a seed crystal for further growth, cutting away areas fromthe end of said stripto leave only the selected crystal connected to themain portion of said strip by a neck portion of the s1l1c on steelmaterial, heating the neck portion and 1n successive" operations bendingtheneck portion about an axis parallel to the plane of the strip and anax1s perpendicular to the plane of the. strip to bring the selected seedcrystal a temperature between about 700 C. and 1000 C. in a reducingatmosphere. I

2. The method of claim 1 wherein the final cold rolling and.recrystallizing treatment is repeated.

3. The method of producing silicon steel magnetic strip material havinga high degree of preferred magnetic orientation'which comprisesisolating a crystal at one end of a silicon steel strip, cutting awaythe crystals from said one end to leave said crystal connected by a thin'neckto the remainder of said strip, orienting the crystallo graphicdirectionsof the crystal by plastically deforming the neck at redv heatuntil said crystal has a '[001] orientation with reference to the mainportion of said, strip,.introducing the end of the strip including saidsee d crystal into a temperature gradient furnace to heat said;

strip to a temperature of 900fC.-1000- C., moving said strip intosaidfurnace at ,a rate; equal to the rate; of;

growth of said-,seed crystal, cold rolling said strip .-to

bring about a reductionin thicknessof -abo ut 70 percent,and-recrystallizingsaid-strip at a temperature-be It was found that thematrix produced by the above treatment of such a steel would support thegrowth of crystals havingorientations, onlynear (110) [001] at atemperature of 900 C.- 1000 C. when fed. into the gradient temperaturefurnace.

5 tvlvleen about 700 C. and 1000" C. in a reducing atmosp ere.

4. The method of claim 3 in which the reducing atmosphere is hydrogen.

5. The method of producing silicon steel magnetic strip material havinga high degree of preferred magnetic orientation which comprisesisolating a single crystal at one end of a silicon steel strip bycutting away other crystals from said one end to leave said crystalconnected by a thin neck to the remainder of said strip, orienting thecrystallographic directions of the crystal by plastically deforming theneck at red heat until said crystal has a (100) [001] orientation withreference to the main portion of said strip, introducing the end of thestrip including said seed crystal into a temperature gradient furnace toheat said strip to a temperature of 900 C. -1000 0., moving said stripinto said furnace at a rate equal 6 to the rate of growth of said seedcrystal, subjecting said strip to a treatment consisting of cold rollingsaid strip to reduce its thickness by at least 70 per cent followed byheating said strip to a recrystallizing temperature of about 980 C. in areducing atmosphere, and repeating said treatment at least once.

Journal of Science of Hirosima University, vol. 11, March 1941, pages89-92, and vol. 9, 1939, pages 227- 231.

1. THE METHOD OF PRODUCING THIN ORIENTED MAGNETIC STRIPS WHICH COMPRISESCOLD ROLLING A SILICON STEEL STRIP OF AN INITIAL THICKNESS OF ABOUT0.005 TO 0.025 INCH BY A TWO-STAGE PROCESS WITH AN INTERMEDIATE ANNEALAT 850* C. BETWEEN STAGES, HEATING ONE END OF THE COLD ROLLED SILICONSTEEL STRIP TO GROW A NUMBER OF CRYSTALS IN SAID END, SELECTING ONE OFSAID CRYSTALS AS A SEED CRYSTAL FOR FURTHER GROWTH, CUTTING AWAY AREASFROM THE END OF SAID STRIP TO LEAVE ONLY THE SELECTED CRYSTAL CONNECTEDTO THE MAIN PORTION OF SAID STRIP BY A NECK PORTION OF THE SILICON STEELMATERIAL, HEATING THE NECK PORTION AND IN SUCCESSIVE OPERATIONS BENDINGTHE NECK PORTION ABOUT AN AXIS PARALLEL TO THE PLANE OF THE STRIP AND ANAXIS PERPENDICULAR TO THE PLANE OF THE STRIP TO BRING THE SELECTED SEEDCRYSTAL INTO (110) (001) ORIENTATION WITH REFERENCE TO THE MAIN PORTIONOF SAID STRIP, INTRODUCING THE END OF THE STRIP INCLUDING SAID SEEDCRYSTAL INTO A TEMPERATURE GRADIENT FURNACE TO HEAT SAID STRIP TO ATEMPERATURE OF 900* C.100* C., MOVING SAID STRIP INTO SAID FURNANCE AT ARATE EQUAL TO THE RATE OF GROWTH OF SAID SEED CRYSTAL, FURTHER COLDROLLING SAID STRIP TO BRING ABOUT A REDUCTION IN THICKNESS OF ABOUT 70PER CENT, AND RECRYSTALLIZING SAID STRIP AT A TEMPERATURE BETWEEN ABOUT700* C. AND 1000* C. IN A REDUCING ATMOSPHERE.